Multi-color microcavity resonant display

ABSTRACT

A resonant microcavity display ( 20 ) having microcavity with a substrate ( 25 ), a phosphor active region ( 50 ) and front and rear reflectors ( 30  and  60 ). The front and rear reflectors may be spaced to create either a standing or traveling electromagnetic wave to enhance the efficiency of the light transmission.

This application is a continuation of Ser. No. 09/628,490, filed Jul.31, 2000, which is a divisional of Ser. No. 09/073,711, filed May 6,1998, now U.S. Pat. No. 6,198,211, which is a divisional of Ser. No.08/581,622 filed Jan. 18, 1996, now U.S. Pat. No. 5,804,919, which is acontinuation-in-part of Ser. No. 08/094,767 filed Jul. 20, 1993, nowU.S. Pat. No. 5,469,018, based upon and claiming priority from PCTapplication No. PCT/US94/08306 filed on Jul. 20, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a luminescent screen comprising aresonant microcavity having a phosphor active region.

2. Description of the Prior Art

Conventional cathode ray tube (CRT) displays use electrons emitted froman electron gun and accelerate them through an intense electric fieldprojecting them onto a screen coated with a phosphor material in theform of a powder. The high-energy electrons excite luminescence centersin the phosphors which emit visible light uniformly in all directions.CRT's are well established in the prior art and are commonly found intelevision picture tubes, computer monitors and many other devices.

Displays using powder phosphors suffer from several significantlimitations, including: low directional luminosity (i.e., brightness inone direction) relative to the power consumed; poor heat transfer anddissipation characteristics; and a limited selection of phosphorchromaticities (i.e., the colors of the light emanating from the excitedphosphors).

The directional luminosity is an important feature of a display becausethe directional properties influence the efficiency with which it can beeffectively coupled to other devices (e.g., lenses for projectionCRT's). The normal light flux pattern observed from a luminescent screenclosely follows a “Lambertian distribution”; i.e., light is emitteduniformly in all direction. For direct viewing purposes this isdesirable, as the picture can be seen from all viewing angles. However,for certain applications a Lambertian distribution of the light flux isinefficient. These applications include projection displays and thetransferring of images to detectors for subsequent image processing.

Heat transfer and dissipation characteristics are important because oneof the limiting factors in obtaining bright CRT's suitable for largescreen projection is the heating of the phosphor screen. As the incidentelectron beam density increases, the phosphor temperature increases.When the phosphor reaches a certain temperature, its luminositydecreases. This is known as thermal quenching. With conventionalpowder-phosphor displays the phosphor-to-screen heat transfercharacteristics are relatively poor, therefore heat dissipation islimited and thermal quenching can occur at relatively low electron beamdensities. Because projection displays require high electron beamdensities to produce the brightness required to project an image, thisinefficiency makes conventional CRT's poorly suited for projectiondisplays.

Chromaticity is important because the faithful reproduction of colors ina display requires that the three primary-color phosphors (red, greenand blue) conform to industry chromaticity standards (e.g., EuropeanBroadcasting Union specifications). Finding phosphors for each of thethree primary colors that exactly match these specifications is one ofthe most troublesome aspects of phosphor development.

The decay time of the activator (i.e., light emitting ion in thephosphor) is also another important parameter for a phosphor. In anideal phosphor for high brightness applications, it is desirable tocontrol directly the decay time of the phosphor for each displayapplication. For example, in some applications, shorter decay timesallow rapid re-excitation of the activator with a corresponding increasein the maximum light output. The decay time is typically determined bythe natural spontaneous transition rate of the activator. In order toimprove phosphor performance it is therefore desirable to have controlover this spontaneous transition rate.

Another problem encountered in conventional phosphor displays is thatenergy can transfer from one activator to another nearby activator inthe phosphor host matrix. This is a nonradiative process where theefficiency of the phosphor is reduced. The energy transfer increaseswith increasing activator concentration and therefore it limits thedensity of activators that can be incorporated in a display and thus themaximum light output.

The use of a single-crystal, thin-film phosphor as a faceplate for a CRTwas first described in a British patent application by M. W. Van Tol, etal., UK Pat. GB-2000173A (1980). This patent taught the use of anyttrium aluminum garnet Y₂Al₅O₁₂ (YAG) film grown by liquid phaseepitaxy (LPE) on a single-crystal YAG substrate. The YAG film is dopedwith a rare-earth ion which emits light when excited by electrons.(Doping is the process wherein dopant ions are substituted for host ionsin the crystal lattice during crystal growth.) In this device, thethickness of the thin-film layer is from one to six microns and does notbear any relation to the wavelength of the light to be emitted by thedisplay.

This device exhibited several advantages over conventionalpowder-phosphor displays. One such advantage was that heat wastransferred from the phosphor more efficiently because of the perfectcontact between the phosphor and the screen, and because of the highthermal conductivity of the YAG substrate. The screen could be loadedwith a higher beam density without exhibiting thermal quenching and,therefore, could produce more light.

Another advantage of single-crystal phosphor luminescent screens versuspowder deposited luminescent screens is concerned with the resolution ofa pixel (i.e., light producing spot). For high resolution displays usingpowder phosphor, the limiting size of a pixel—and hence the resolutionof the screen—is determined by the particle size of the phosphor powder.Single-crystal phosphors, on the other hand, are not affected by thissince they do not contain discrete particles.

Powder phosphors further reduce resolution due to the light scatteringfrom the surface of the powder. Because of the lack of discrete phosphorparticles and the absence of light scattering, thin-film displays havehigh image resolution, limited only by the spot size of the excitingelectron beam. The increasing demand for higher resolution displaysmakes this a particularly attractive advantage.

Yet another advantage is concerned with producing a vacuum in a CRT. Toallow the electron beam to travel between the electron gun and thephosphor screen, a vacuum must be maintained within a CRT. Conventionalpowder phosphors have a high total surface area and, generally, organiccompounds are used in their deposition. Both the high surface area andthe presence of residual organic compounds cause problems in holding andmaintaining a good vacuum in the CRT. Using thin-film phosphorsovercomes both of these effects, as the total external surface area ofthe tube is controlled by the area of the thin-film (which is much lessthan the surface area of a powder phosphor display) and, furthermore,there are no residual organic compounds present in thin-film displays toreduce the vacuum in the sealed tube.

The thin-film phosphors of Van Tol, et al., exhibit one prohibitingdisadvantage, however, due to the phenomenon of “light piping.” Lightpiping is the trapping of light within the thin-film, rendering itincapable of being emitted from the device. This is caused by the totalinternal reflection of the light rays generated within the thin-film.Since the index of refraction (n) of most phosphors is around n=2, onlythose light rays whose incident angles are less than the critical angle,θ_(c) (where sin θ_(c)=1/n) will be emitted from the front of thethin-film. The critical angle for an n=2 material is around 30°.Therefore, the fraction of light that escapes from the front of thethin-film is only about 6.7% of the total light. The common design ofplacing a highly reflective aluminum layer behind the film only doublesthe output to about 13% of the light. Moreover, this light is spread ina “Lambertian distribution” and is not directional. As a result of lightpiping, the external efficiency (i.e., the percentage of photonsescaping from the display relative to all photons created in thedisplay) is less than one-tenth that of powder phosphor displays.Therefore, in spite of the unique advantages offered in terms of thermalproperties, resolution, and vacuum maintenance; the development ofcommercial CRT devices based on thin-films is held back by their poorefficiency due to “light piping”.

Some schemes have been designed to reduce the “light piping” problem.One scheme described by Bongers, et al., U.S. Pat. No. 4,298,820 (1981),uses a thin-film, deposited by LPE, with V-shaped grooves etched intothe surface to reflect light out of the thin-film. This approach broughtabout an improvement in external efficiency of around 1½ to 2½ timesthat of a thin-film display without the V-shaped grooves. Given theprevious external efficiency of 13%, this would still only lead to atotal external efficiency of around 20% to 30%.

Another scheme, described by Huo and Hou, “Reticulated Single-CrystalLuminescent Screen”, 133 J. Electrochem. Soc. 1492 (1986), involvesetching individual mesa shapes onto the thin-film deposited by LPE. Thisled to a three times improvement in external efficiency (still renderingonly about a 30% external efficiency). Furthermore, since the phosphorlayer was no longer smooth, any light rays that were internallyreflected could find themselves rescattered to areas far from theirpoint of creation, thus spoiling the resolution of the display.

Microcavity resonators, which can be incorporated in the presentinvention, have existed for some time and have recently been describedby H. Yokoyama, “Physics and Device Applications of OpticalMicrocavities” 256 Science 66 (1992). Microcavities are one example of ageneral structure that has the unique ability to control the decay rate,the directional characteristics and the frequency characteristics ofluminescence centers located within them. The changes in the opticalbehavior of the luminescence centers involve modification of thefundamental mechanisms of spontaneous and stimulated emission.Physically, such structures as microcavities are optical resonantcavities with dimensions ranging from less than one wavelength of lightup to tens of wavelengths. These have been typically formed as oneintegrated structure using thin-film technology. Microcavities involvingplanar, as well as hemispherical, reflectors have been constructed forlaser applications.

Resonant microcavities with semiconductor active layers, for examplesilicon or GaAs, have been developed as semiconductor lasers and aslight-emitting diodes (LEDs).

E. F. Schubert, et al., “Giant Enhancement of Luminescence Intensity inEr-doped Si/SiO2 Resonant Microcavities” 61(12) Appl. Phys. Lett. 1381(1992), describes a resonant microcavity with an Er doped SiO₂ activelayer. This device emits radiation in the infrared region and isintended as a laser amplifier for fiber-optic communications.

The Schubert device, the semiconductor lasers and the LEDs are not assuitable for use in luminescent displays for several reasons. Theycontain luminescent materials such as Si, GaAs, etc., in the activeregion which are suitable as laser media, but which are typicallyinefficient emitters of visible light and require excitation by theinjection of electrons. They also are designed with small planar surfaceareas that are inadequate for display purposes. Moreover, because of thedesign of these devices and the active materials used, they typicallycannot be excited efficiently with electron bombardment, an electricfield, or ultraviolet radiation. These excitation mechanisms are anessential part of the current display technologies.

Furthermore, the laser microcavity devices work above the laserthreshold, with the result that their response is inherently nonlinearnear this threshold and their brightness is limited to a narrow dynamicrange. Displays, conversely, require a wide dynamic range of brightness.Microcavity lasers utilize stimulated emission and not spontaneousemission. As a result, these devices produce highly coherent lightmaking these devices less suitable for use in displays. Highly coherentlight exhibits a phenomenon called speckle. When viewed by the eye,highly coherent light appears as a pattern of alternating bright anddark regions of various sizes. To produce clear, images, luminescentdisplays must produce incoherent light.

In addition, it is important to distinguish the resonant microcavitydisplay from the laser CRT. This display is similar to a CRT and scansan electron beam to write the information to the luminescent screen.However, the light is not produced by the spontaneous emission of thephosphor, but by stimulated emission. The faceplate of the laser CRT isan electron beam pumped semiconductor laser. The active medium, asemiconductor, is placed between two mirrors that form a laser cavity.The cavity structure is contained within the faceplate. When pumped witha sufficiently energetic electron beam, the device lases, producing ahighly energetic and directional light beam. Such a display is describedby A. S. Nasibov, et. al. in the article “Full Color TV projector basedon A₂B₆ electron-pumped semiconductor lasers”, J. Crystal Growth, 117,1040 (1992).

SUMMARY OF THE INVENTION

The subject invention, the Resonant Microcavity Display (RMD), is aluminescent display which offers the advantages of a thin-film phosphorwithout exhibiting the light piping problem. This is because it emitslight in a highly directional manner as a result of its geometry.

The resonant microcavity display is any structure that modifiesspontaneous emission properties of a phosphor contained within thestructure. The modification of spontaneous emission is obtained bychanging the optical mode amplitudes to the such a degree that thephosphor favorably emits into a relatively few optical modes. It is alsopossible to suppress emission in certain optical modes. Thismodification of mode amplitudes can be created, for example, by theformation of a standing wave electric field for each favored mode withinthe structure and locating the phosphor at the antinodes of thesestanding waves. It is essential that the standing waves havesubstantially modified electric field amplitudes relative to the thefield amplitudes generated without a cavity. Substantially modifiedrefers to changes by a factor of two or more in the field amplitudes.

In standing wave cavities, no enhancement can occur at the node of theelectric field. However, a ring cavity design 320 such as that shown inthe downward-looking view of FIG. 1 supports a traveling wave 322 inwhich the electric field amplitude is substantially modified throughoutthe entire cavity. As a result, mode enhancement or suppression canoccur throughout the cavity. Compared to the standing wave cavity, moreactive medium 324 with modified light emission can be utilized for thesame cavity volume.

One example of a resonant microcavity display is a microcavity resonatorcomprising a phosphor sandwiched between two reflectors, all of whichare grown on a transparent rigid substrate. The width of the activeregion is chosen such that a resonant standing wave, of the wavelengthto be emitted, is produced between the two reflectors. In its simplestform, a single coplanar microcavity, the two reflectors are parallel toeach other and the plane of the active region is parallel to thereflectors. Other geometries which produce standing waves or travelingwaves with an increased electric field amplitude, such as combinationsof planar microcavities, three-dimensional microcavities, confocalmicrocavities, hemispherical microcavities, or ring cavities are alsopossible. These other geometries are well-known in the art of designingcavities.

Another structure that favorably alters the spontaneous emissionproperties uses photonic band gap crystals. A photonic band gap crystalcan be formed from a monodispersed colloidal suspension. The structurescomprise periodic dielectric media to create a band gap of energy forwhich light cannot propagate within the structure. However, doping sucha structure with a material that has a resonance within the band gapwill create a high Q cavity. Such cavities can be one, two or threedimensional. The cavity generates a standing wave with an enhancedelectric field amplitude in the region of the dopant. In order to createa display, the photonic band gap crystals must be a phosphor. Henry O.Everitt describes photonic band gap crystals in “Applications ofPhotonic Band Gap Structures”, Optics & Photonics News, 20, (1992). FIG.2 is a side view of a resonant microcavity display 350 on a substrate352 using a photonic band gap crystal 354 as the entire cavitystructure.

Fabricating the RMD requires the use of a growth technique capable ofcontrolling layer thickness or the spatial resolution of the refractiveindex to a precision of several nanometers. Such techniques, forexample, include, but are not limited to, chemical vapor deposition(CVD), molecular beam epitaxy (MBE), atomic layer epitaxy (ALE),electron beam evaporation, or sputtering. Fabricating the RMD may alsoemploy holographic photo-lithographic techniques. In this case, theBragg reflectors are created by exposing a suitable material to aholographic pattern thereby creating in the material alternating layersof high and low refractive index regions. Such a technique is well knownin the art of fabricating holographic diffraction gratings.

The substrate can be either a crystalline, polymer, or an amorphoussolid. It can be made of any material that will allow the other regionsto be grown on it. Suitable substrate materials may be chosen from awide range of materials such as oxides, fluorides, aluminates, andsilicates. The substrate material can also be fabricated using organicmaterials. The criteria involved in selecting a substrate materialinclude its thermal conductivity and its compatibility (both physicaland chemical) with other materials forming the RMD.

The phosphor may be excited through several means, including:bombardment by externally generated electrons (cathodoluminescence),excitation by electrodes placed across the active layer to create anelectric field (electroluminescence), or excitation using photons(photoluminescence).

The present invention is distinguished from other microcavity devices inpart by the placing of a phosphor in the resonant microcavity. Phosphorsare materials that exhibit superior visible luminous efficiencies (whereluminous efficiency, as used herein, is defined as the ratio of lightoutput in Watts over the power input in Watts). Typically, the luminousefficiencies of phosphors range between 1% and 20%. These highefficiency materials are only classified as phosphors if the materialefficiently generates luminescence when excited by electrons, electricfields, or light.

The active region may comprise a wide range of inorganic phosphors(e.g., sulfides, oxides, silicates, oxysulfides, and aluminates) mostcommonly activated with transition metals, rare earths or color centers.In addition to inorganic phosphors, the active region may employ anorganic phosphor such as tris (8-hydroxyquinoline) aluminum complex. Theactive region comprises phosphors typically in the form of singlecrystal films, polycrystalline films, amorphous films, thin powderlayers, liquids, or some combination of the above. A selection ofphosphors that have found commercial applications, and from which anapplication dependent phosphor can typically be selected for use in thepresent invention, is documented in “Optical Characteristics of CathodeRay Tube Screens,” Electronic Industries Association Publication TEP116.

The reflectors forming the resonant cavity consist of either metalliclayers or Bragg reflectors. Bragg reflectors are dielectric reflectorsformed from alternating layers of materials with differing indices ofrefraction. The simplest geometry for dielectric reflectors consists ofone-quarter wavelength thick layers of a low refractive index material,such as a fluoride or certain oxides, alternating with one-quarterwavelength thick layers of a high refractive index material, such as asulfide, selenide, nitride, or certain oxides. The dielectric reflectorscan also be fabricated using organic materials. Mirrors can also beformed using photonic band gap crystals. Any incident light with anenergy within the band gap will be reflected by the structure. FIG. 3shows a side view of an illustrative embodiment of a resonantmicrocavity display 340 on a substrate 342 in which an active layer 346is sandwiched between two mirrors 344, 348 comprising photonic band gapcrystals.

In current display applications, only one side of the screen is viewed.In the case of a microcavity, the design requires the use of differentreflectors in order for most of the light to be projected towards theviewer. In the case of the simple coplanar microcavity, this asymmetryis obtained by having one of the two reflectors be substantially whollyreflective, meaning that it reflects most of the light impinging on it.The other reflector (opposite to the substantially wholly-refractivereflector) is partially reflective, meaning that it does not reflect ashigh of a percentage of impinging light as the wholly-reflectivereflector and allows some of the light to pass through it. Because ofthe difference in reflectance of the two reflectors, virtually all ofthe light produced in the active region escapes through thepartially-reflective reflector along the axis normal to the plane of thedevice.

In the case of a microcavity structure, the dimensions depend on thenatural spontaneous emission spectrum of the phosphor being used, asobserved outside of a cavity. If the spectrum covers a broad range ofvisible wavelengths it is possible to choose an appropriate part of thespectrum (i.e., one that matches an industry standard chromaticity) andconstruct the microcavity with a matching resonance. The finalchromaticity of the RMD will correspond to the cavity resonance and willbe different from the natural chromaticity of the phosphor outside ofthe microcavity. Conversely, if the phosphor's natural spontaneousemission spectrum covers only a narrow range of visible wavelengths, thedimensions would be chosen so that the cavity resonance would match oneof the phosphor's emission bands.

The RMD has a highly directional light output similar to those of aprojector or a flashlight and, as a result, RMDs can be constructed toavoid light piping. This allows highly efficient coupling to otherdevices. RMD's also have a high external efficiency, approaching 100%.Since RMDs incorporate films, RMDs permit the design of efficientthermal conduction of the heat generated in the active layer. Thisfeature combined with the ability to reduce the phosphor decay timeallow RMDs to utilize intense excitation. As a result of the above, RMDsare especially suitable for use in projection displays.

It is therefore an object of this invention to provide a luminescentdisplay that does not exhibit the problem of light piping.

It is a further object of this invention to provide a luminescentdisplay with highly efficient heat transfer properties.

It is a further object of this invention to provide a luminescentdisplay with a high external efficiency.

It is a further object of this invention to provide a luminescentdisplay capable of high resolution.

It is a further object of this invention to provide a luminescentdisplay which produces a highly directional output.

It is a further object of this invention to provide a luminescentdisplay in which the chromaticity of the emitted light can be accuratelycontrolled irrespective of the nature of the phosphor used.

It is a further object of this invention to provide a luminescentdisplay wherein the phosphor used can be chosen to optimize the displaywith respect to properties other than chromaticity.

It is a further object of this invention to provide a luminescentdisplay wherein the decay time of the activator can be tailored for thespecific display application.

It is a further object of this invention to provide a luminescentdisplay which can be heavily loaded by the excitation source withoutsaturating the phosphor due to overheating.

Other objects and advantages of the present invention will becomeapparent to those skilled in the art from the following detaileddescription of the illustrated embodiments, when read in light of theaccompanying drawings.

Throughout this specification, published articles are cited forbackground purposes. These articles are hereby incorporated by referenceinto this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top sectional view of a traveling wave cavity in oneillustrative embodiment of a resonant microcavity display in accordancewith the invention.

FIG. 2 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention using aphotonic band gap crystal as a resonant microcavity display.

FIG. 3 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention usingphotonic band gap crystals as mirrors.

FIG. 4 is a perspective illustration of an illustrative embodiment of aresonant microcavity display in accordance with the invention employinga planar mirror resonator.

FIG. 5 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention employinga confocal resonator.

FIG. 6 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention usingmultiple cavity structures.

FIG. 7 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the inventionincorporating an integral optical element.

FIG. 8 is a perspective view of one illustrative embodiment of aresonant microcavity display in accordance with the invention employingcathodoluminescent excitation.

FIG. 9 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention as itwould be used in a cathode ray tube.

FIG. 10 is a side sectional view of an illustrative experimentalembodiment of a resonant microcavity display in accordance with theinvention designed to emit light through its front reflector with awavelength of 530 nanometers.

FIG. 11 is a graph relating the reflectance of the resonant microcavitydisplay of FIG. 10 as a function of the wavelength of the incidentlight.

FIG. 12 is a side sectional view of a direct view color televisionemploying a resonant microcavity display in accordance with theinvention.

FIG. 13a is a perspective illustration of an array of pixel-sizedmicrocavities as used in a color television in accordance with theinvention.

FIG. 13b is an illustration of a front view of an array of pixel-sizedmicrocavities as used in a color television in accordance with theinvention. The front view shown in FIG. 13b corresponds to a view fromthe top of FIG. 13a.

FIG. 14 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the inventionincorporated in a vacuum fluorescent display.

FIG. 15 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention using anarray of high voltage field emission devices for excitation of itsactive layer.

FIG. 16 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention using alow voltage field emission material for excitation of the active layer.

FIG. 17 is a schematic illustration of the standing wave electric fieldin one illustrative embodiment of a resonant microcavity display inaccordance with the invention.

FIG. 18 is a perspective drawing of an illustrative embodiment of aresonant microcavity display in accordance with the invention excited byan electric field.

FIG. 19 is a perspective drawing of an illustrative embodiment resonantmicrocavity display in accordance with the invention excited withultra-violet light.

FIG. 20 is a side sectional view of an illustrative embodiment of atransparent resonant microcavity display in accordance with theinvention.

FIG. 21 is a schematic illustration of an illustrative embodiment of aresonant microcavity display in accordance with the invention employinga laser for excitation.

FIG. 22 is a schematic illustration of an illustrative embodiment of atunable resonant microcavity display in accordance with the invention.

FIG. 23 is a schematic illustration of an illustrative embodiment of aresonant microcavity display in accordance with the invention used as alight source for a liquid crystal display light valve application.

DETAILED DESCRIPTION OF THE INVENTION

The present invention employs quantum electrodynamic (QED) theory toenhance the properties of the light emitted from phosphor basedluminescence displays. The performance of a given display applicationdepends on properties of the emitted light such as the chromaticity,direction, and flux. These properties can be optimized by employing theprinciples of QED theory in the design of microcavities so as to controlthe spontaneous emission characteristics of the phosphor activator foreach specific display application.

As seen in FIG. 4, one example of the present invention 10 comprises aphosphor embedded in a resonant microcavity 20 grown on a substrate 25.The microcavity 20 further comprises a front reflector 30, aphosphor-based active region 50, and a back reflector 60. The activeregion 50 is disposed between two reflectors 30 and 60. The structuremay comprise a variety of materials and may employ a variety ofresonator designs. FIG. 4 illustrates a planar mirror design, whereasFIG. 5 illustrates the present invention configured in a confocal mirrordesign. The confocal design has the advantage of having an inherentlycavity quality factor (Q).

More complex cavity designs involve stacking multiple microcavities.This design is similar to the standard method for forming interferencedevices which typically consist of 2 or more stacked cavities where eachcavity is separated by a coupling layer. Such structures are used in thefabrication of, for example, bandpass optical filters, narrow bandoptical reflectors and long wavelength or short wavelength cutofffilters.

The invention can only be completely understood by employing quantumelectrodynamic (QED) theory as applied to a cavity. Cavity QEDcalculations allow one to determine the following parameters for a givendegree of activator excitation and activator concentration: the amountof light emitted from the microcavity; the angular spread of the lightemitted; and the color of the light emitted.

The calculation begins by determining the nature of the electromagneticfield inside and outside of the cavity. This field calculation usesMaxwell's equations with the boundary conditions imposed by themicrocavity. Applying Fourier analysis, the net electromagnetic field isbroken down into its fundamental constituents, the optical modes.

An optical mode is a field with a characteristic frequency, directionand polarization. The square of the field intensity corresponds to theactual amount of light. One must select from this field distributionthose optical modes that correspond to useful light. For a display,useful light is defined as any light emitted from the cavity within acertain predetermined angular spatial distribution and predeterminedfrequency spread.

The next step is to calculate the amount of light emitted by eachactivator. This calculation begins by determining the radiative decayrate of each activator for each possible optical mode. The radiativedecay consists of a spontaneous emission rate and a stimulated emissionrate. The resonant microcavity display, however, only operatessatisfactorily as a display when there is no stimulated emission (i.e.,constructing a microcavity to operate as a laser would preclude using itas a display). The degree of excitation, the type and concentration ofthe activators and the resonator design determines when stimulatedemission is an issue.

The spontaneous emission rate is determined by using QED theory tocalculate the probability that a single excited activator will decayinto a specific optical mode. This calculation must use the fieldstrength appropriate for the location of the activator in the cavity.The magnitude of the standing or traveling wave within the cavity mayhave different values throughout the phosphor layer. In addition, acertain probability exists that each excited activator will decaywithout emitting light. To calculate this non-radiative rate, one mustconsider cavity QED effects as they apply to the physical mechanismresponsible for the non-radiative decay.

For a given excitation level, one can now calculate the amount ofspontaneous emitted light for each activator. The ratio of thespontaneous rate to the sum of the radiative and non-radiative ratesyields the percentage of excitation that will produce light. The amountof useful light is then determined by calculating the amount of thespontaneous emission in the desired optical modes. This calculation isperformed for each activator. Finally, the sum of all the activatorcontributions yields the display intensity of the RMD.

The properties of the RMD that can be controlled include thechromaticity, the directionality of the display, the luminous efficiencyand the maximum light output of the display. These properties are tunedaccording to the requirements of the specific luminescent screenapplication. The parameters that must be considered for optimization arethe microcavity Q, the microcavity resonance frequency, the asymmetry ofthe reflectors, the resonator design (i.e., planar, confocal, multiplecavity, etc.), the phosphor, the thickness of the phosphor layer, thesurface area of the microcavity and the excitation source. Theseparameters cannot be optimized separately; each affects the otheradjustable properties of the display.

The performance of the resonant microcavity can be described by the Q ofthe cavity. The Q of the cavity is given by the microcavity centerfrequency divided by the linewidth of the microcavity resonance:$Q = \frac{\nu}{\Delta \quad \nu}$

where ν is the microcavity resonance frequency and Δν is the linewidthof the cavity resonance. The cavity Q is determined primarily by thereflectance of the reflectors, the resonator design, the asymmetry inthe reflectance and any imperfections in the cavity. These imperfectionstypically result from defects in the structure of the resonantmicrocavity which scatter light out of the cavity in a non-usefulmanner. The Q can be measured empirically using an optical spectrometer.

As the cavity Q increases, the display brightness and efficiencyincreases. In addition, the angular spread of the light decreases andthe linewidth shrinks altering the chromaticity. Note that as thespatial distribution of the light narrows, the amount of light incertain regions decreases. Depending on the display application, thiseffect may or may not be desirable. For the range of the current displayapplications, the engineered cavity Q will typically vary between 10 and10,000. The above effects can be determined experimentally by measuringthe light intensity as a function of solid angle for resonantmicrocavities with different Q values. Using this data, one can predictthe required Q for a given application.

For most current applications, only one side of the luminescence screenis viewed. In these applications one should choose reflectors withdifferent reflectivities such that the display preferentially forces thelight out the cavity towards the viewer.

The resonator design directly affect the Q and mode volume. The latterterm describes the actual volume of the activator layer that isparticipating in producing useful light. This volume is related to thespatial distribution of the electromagnetic field within the activatorlayer. The design of the resonator will also determine the spatialdistribution of useful light. Due to the relatively straightforwardconstruction, the simplest design is a planar resonator. However, otherresonator structures which produce standing waves or traveling waveswith an enhanced electric field intensity in a phosphor material may beuseful. In particular, multiple planar microcavities may be combined toallow for a larger active region or to achieve greater control over theallowed emission than can be achieved with a single cavity.

FIG. 6 provides one illustrative design for a 3 cavity resonantmicrocavity 200. In this example, each cavity 201A, 201B, 201C comprisesdielectric reflectors 202, 206. The dielectric reflectors 202, 206 areseparated by a half-wavelength coupling layer 204 in cavities 201A and201B, while adjacent cavities are separated from each other by ahalf-wavelength spacer 208. The phosphor material 209 is also ahalf-wavelength thick and is located within the lowest cavity, takingthe place of a half-wavelength coupling layer in cavity 201C. Thedistances specified are optical thicknesses, i.e. the index ofrefraction multiplied by the physical thickness of the layer.

As already discussed in the case of the planar geometry, there exists anentire set of parameters to consider including the individual mirrorreflectances and individual cavity Q's. In addition, one must alsodetermine the cavity spacing, coupling layer, and the location of thephosphor material. The exact specifications will depend on the specificdisplay requirements.

A primary design specification of the RMD is the chromaticity of theemitted light. The center frequency and linewidth of the cavity must beengineered so that the RMD displays this color of light.

Once these parameters are selected, the phosphor must be selected. Thephosphor will need to have a natural luminescence resonance thatoverlaps the cavity resonance. As the resonance narrows and the overlapincreases, the display efficiency and brightness increase. A compromisebetween chromaticity and other parameters may be required to optimize adisplay for a specific application.

The intensity of light emitted by the phosphor is related to theactivator concentration: as the concentration increases, the intensityof emitted light increases. The activator concentration, however, isoften limited by non-radiative energy transfer between activators thatquenches luminescence. These quenching effects are concentrationdependent. The quenching concentration varies from phosphor to phosphor,depending on the magnitude of various energy transfer parameters betweenactivators. Cavity QED theory predicts that there is an effect on theseparameters since they relate to spontaneous emission characteristics.Thus, another potential advantage of the RMD is that energy transferbetween activators may be suppressed and phosphors could contain higherconcentrations of activators than was previously possible, withoutlosing efficiency. In addition, phosphors can simultaneously emitseveral wavelengths corresponding to different optical transitionswithin the material. However, only one of these transitions typicallygenerates the useful light of the display. A microcavity can be designedto enhance this useful transition while inhibiting the non-usefultransition(s). This suppression will increase the efficiency of thedisplay. The ability of a structure to inhibit spontaneous emission andenergy transfer processes has been described by G. Kurizki and A. Z.Genack in “Suppression of Molecular Interactions in Periodic DielectricStructures”, Phy. Rev. Let. 61, 2269 (1988).

The display properties also depend upon the thickness of the activeregion. Depending on the cavity design, there may be several activeregion thicknesses that produce a predetermined frequency. The range ofthickness depends on the mirror construction. As the thicknessincreases, the number of potentially excited activators increases. Withsufficient excitation energy, the total luminescence can be increasedwith a wider active region. However, the thickness may alter the spatialdistribution in a highly complex manner. In the case of a simplecoplanar microcavity, the angular spread of the light changes, withadditional regions of high intensity appearing at angles that are notnormal to the plane of the microcavity. More complex multiple cavitydesigns allow a greater degree of control over the directionality of thedisplay.

Another key parameter in the resonant microcavity design is the area ofthe emitting surface. Some applications will require one large-areasurface for the production of monochromatic light, while other designswill need pixel-sized cavities capable of producing red, green and bluelight. The size of the pixel will be determined by the resolutionrequirements of the display.

One other important parameter is the excitation source and intensity.The display application will dictate the excitation source. The decisionprocess in selecting the phosphor must also consider the efficiency ofconverting the excitation energy into useful luminescence. Thisefficiency is well documented for the registered phosphors, but caneasily be experimentally determined. The intensity of the source willprimarily change the brightness.

It should be noted that in considering the above design parameters, thelight properties of the display must not reach the degree of coherenceassociated with a laser. To avoid this problem, particular attentionmust be paid to the cavity Q, the activator concentration and theexcitation intensity.

The RMD design lends itself to the incorporation of an optical element382, such as a lens or a diffuser, fabricated within or on top of thesubstrate 384 of a resonant microcavity 386, as shown in FIG. 7. Forexample, a lens would be useful to modify the angular distribution ofthe light output produced by the structure and thereby generate therequired distribution. The lens may be formed using photo-etchingmethods, which is well known in the art of miniature semiconductorlasers. Another method would employ the controlled placement ofimpurities to change the local refractive index. This method is used toconstruct gradient refractive index lenses which are commonly used infiber optics.

Using such a lens adds another parameter that must be considered in theoptimization of the display. However, such a lens enables one tomaximize the output of the resonant microcavity without having toconsider the required light distribution. For example, such a lens wouldeliminate or reduce the demands for the complex lens design currentlyrequired in the projection CRT display applications.

Similarly, a diffuser can be used to precisely control the angularspread of the light and thereby the field of view of the display. Withthe ability to control the light distribution independent of themicrocavity, the spontaneous emission properties of the phosphor can bemaximized without having to consider the required light distribution. Adiffuser can be made using holographic techniques, ruled gratingtechniquess, introduction of internal scattering centers, or preciselycontrolled surface roughening.

The RMD can be embodied using cathodoluminescence which results from anelectron beam bombardment of the phosphor. One example of a device whichemploys cathodoluminescence is a projection television. This applicationrequires the highest intensities possible because it requires a wideviewing area and uses a light dispersing screen. In this application,the resonant microcavity display is incorporated in a CRT.

Full color projection televisions require three separate CRT's: one foreach primary color. In this application, the RMD is superior toconventional methods because it allows intense excitation loading of thephosphor, highly directional output, controlled chromaticity, and highexternal efficiency. Therefore the RMD allows the use of relativelycompact CRT's while maintaining high luminescence.

In the case of a resonant microcavity display incorporated in a CRT, thephosphor is excited by electrons emitted from the electron gun,accelerated to a speed such that most of them will penetrate theresonant microcavity to the depth of the phosphor. The high energyelectrons excite electrons in the phosphor from the valence band intothe conduction band. This additional energy is trapped at the impurity.The impurity then relaxes by emitting visible light.

In the case of a simple coplanar microcavity, the reflectors can beeither dielectric or metallic. The back reflector has a higherreflectivity than the front reflector, so that light, emitted by thephosphor, exits the cavity through the front reflector, perpendicular tothe plane of the thin film device. The microcavity Q and the asymmetryin the reflectance determines the percentage of light that exists theresonator through the front reflector.

In the case of the simple coplanar microcavity, the width of the activeregion affects the directionality of the light and is chosen so that itsoptical path length, i.e. the product of the distance between the backreflector and the front reflector and the index of refraction of thephosphor material, equals an integer multiple of the desired wavelengthdivided by 2 to 4 depending on the index of the adjacent layers. Thesedimensions ensure that a standing wave builds up between theback-reflector and the front reflector. The wavelength of the emittedlight is determined by the resonant wavelength of the microcavity.

A dielectric, or Bragg, reflector consists of alternating layers ofmaterial with high and low indices of refraction. The number of layersdetermines the reflectivity of the reflector. The reflectivity (R) ofthe reflectors can be calculated using the following equation:$R = \frac{1 - {\left( \frac{n_{H}}{n_{L}} \right)^{N - 1} \times \frac{n_{H}^{2}}{n_{S}}}}{1 + {\left( \frac{n_{H}}{n_{L}} \right)^{N - 1} \times \frac{n_{H}^{2}}{n_{S}}}}$

where n_(H) and n_(L) are the refractive index of the high and low indexof refraction material, respectively; n_(S) is the index of refractionof the substrate and N is the total number of layers in the stack. Thisequation is valid for normal incidence. The width of each layer is equalto an odd integer multiplied by the desired wavelength of light to beemitted divided by the quantity 4 times the index of refraction of thematerial used in the layer. An alternate design uses holographictechniques to form the reflectors. In this case, the mirror is formedfrom one material with a continuously varying refractive index.Photo-lithography would be used to fabricate the mirrors.

The Q of the cavity can be calculated once the reflectivity isdetermined for the reflectors. In the case of the simple coplanarmicrocavity, the equation that relates Q to reflectivity is given by:$Q = \frac{2\quad \pi \quad n\quad \nu}{c\quad \left( {\alpha - {\frac{1}{l}\left( {\ln \sqrt{R_{1}R_{2}}} \right)}} \right.}$

where ν is the microcavity resonance frequency, n is the index ofrefraction of the phosphor, α is the average distributed loss constant,l is the width of the activator layer, R₁ is the reflectance of thefront mirror and R₂ is the reflectance of the back mirror. The constantα is needed to account for the non-ideal behavior of the cavity thatresults from imperfections and spurious absorption.

The parameters chosen to optimize this display depend on the requiredbrightness of the display and the required directionality of the lightoutput. In the typical projection television application, the displayshould be highly directional and bright. For each color, the cavity Qcan be optimized empirically by measuring the total intensity emitted inthe useful direction as a function of the electron beam current. Thisefficiency measurement is common in the television design art.

FIG. 8 shows one illustrative embodiment designed forcathodoluminescence, the simple planar resonant microcavity. The subjectinvention 10 comprises a resonant microcavity 20 grown on a rigidtransparent substrate 25. A layer of aluminum 80 is disposed next to themicrocavity 20 to channel off electrons deposited by the electron beamand to provide an additional reflective surface. The resonantmicrocavity 20 is grown onto the substrate 25 using molecular beamepitaxy (MBE) or any suitable method of solid-state fabrication. Somemethods of growth known to the art (e.g., LPE at its current level ofdevelopment) are not suitable because they cannot be controlled with theprecision necessary to grow a correctly sized microcavity. The activeregion 50 is excited by electrons from an electron beam 54 enteringthrough the aluminum layer 80 and back reflector 60. The light 58created in the active region exists through the front reflector 30 andthe substrate 25.

As seen in FIG. 9, this embodiment can be embodied in a cathode ray tube(CRT) 100 comprising a glass vacuum tube 105 enclosing an electron gun(which is a means to generate an electron beam) 110 aimed at a flatviewing surface 115 and distal from the electron gun 110; and aphosphor-based resonant microcavity 20 disposed parallel to the flatviewing surface 115 inside the vacuum tube 105. This embodiment isconfigured to produce monochromatic light.

As shown in FIG. 10, an experimental embodiment designed to emit lightthrough the front reflector with a wavelength of 530 nanometers, thematerial used in the active region 50 is zinc sulfide (ZnS) doped withmanganese (Mn) at a dopant concentration of 2%. The thickness of theactive region 50 is 110 nanometers and the phosphor has an index ofrefraction of n=2.4.

In the front reflector 30, the material used in the layers with arelatively high index of refraction 32, 36, 40 and 44 is ZnS, and thematerial used in layers with a relatively low index of refraction 34,38, 42 and 46 is calcium fluoride (CaF₂). In the back reflector 60, thematerial used in the layers with a relatively low index of refraction62, 66, 70, 74, 77, and 79 is CaF₂, and the material used in the layerswith a relatively high index of refraction 64, 68, 72, 76, and 78 isZnS. All of the high-index ZnS layers are 55 nanometers thick with anindex of refraction of n=2.4. All of the low-index CaF₂ layers are 95nanometers thick with an index of refraction of n=1.4.

The substrate 25 is made of CaF₂. It is 2 millimeters thick and has anindex of refraction of n=1.4. The aluminum layer 80 is 50 nanometersthick.

The microcavity 20 is grown on the substrate 25 using MBE and thealuminum layer 80 is deposited using vapor-phase deposition.

The front reflector has a reflectivity of R=97.5% with 8 layers and theback reflector has a reflectivity R=99.9% with 12 layers including thealuminum layer. Because the back reflector is more reflective than thefront mirror almost all of light produced in the cavity exists throughthe front reflector.

As shown in FIG. 11, the reflectance of the RMD is a function of thewavelength of the incident light. At the resonance wavelength of 530 nm,the reflectance dips to roughly 86%—indicating that the RMD willtransmit this wavelength. At all other wavelengths the reflectance isnear 100%—indicating that the RMD will not transmit light atnon-resonance wavelengths. This reflectance behavior is due to the factthat the cavity can only support a standing wave of a wavelength equalto the resonance wavelength of the cavity.

In another embodiment, the RMD can be used in a CRT as a direct viewtelevision. FIG. 12 depicts a direct view color television. The CRT 120is similar to the one described in the projection television embodiment,except that it has three electron guns, 122, 124 and 126 one for eachprimary color. Each of the electron guns produces a separate electronbeam, 130, 132 and 134, corresponding to the desired intensity of eachcolor. The electron beams excite a screen 140 on the viewing surface ofthe CRT.

As seen in FIG. 13a, the screen 140 comprises of an array of pixel-sizedmicrocavities 20. The array contains microcavities designed to producered light 142, green light 144 and blue light 146. The red-light pixelsare excited by the “red” electron beam 130, the green-light pixels areexcited by the “green” electron beam 132, and the blue-light pixels areexcited by the “blue” electron beam 134. FIG. 13b shows a front view ofthe array of pixels and the arrangement of colors. The design of colordisplays with separate color pixels is well known in the art.

In this embodiment, the light emanating from the pixel produces therequired angular distribution. One could also envision an embodiment inwhich a lens is used to achieve this display requirement allowing forthe maximum efficiency to be produced by the resonant microcavity. Therequired angular distribution can also be obtained using a diffuser suchas a holographic optical element.

The construction of the pixel is fundamentally the same as thatdescribed in the embodiment for a projection television. The primarydifference is the size of the surface area and the angular spread oflight required. In this case, the surface area is determined not bybrightness, but by the resolution required by the application. Highdefinition television, medical and military applications typicallyrequire the pixel size to be smaller than 25 microns. This requirementis difficult to achieve using current technologies, but can be easilyachieved using an RMD.

With the resolution and angular distribution specified, the resonantmicrocavity display must be optimized for each color. This optimizationwill use the above-described empirical method of measuring the totallight produced versus beam current. The restrictions of the design dueto the specification mean that obtaining the maximum light output isprimarily a function of the phosphor activator. In the embodiment inwhich a lens is placed outside the cavity, one has much more freedom inengineering the cavity. Without the restriction on the angulardistribution, the cavity Q can be easily tailored.

In another embodiment using electrons that excite the active layer, theresonant microcavity 217 can be incorporated in a vacuum fluorescentdisplay 210, as shown in FIG. 14. Display 210 comprises individualpixels which are typically combined to form low resolution, compactinformation displays and extremely large displays.

A vacuum fluorescent display generally comprises an array of cathodes226, a control grid 224 and phosphor coated anodes, corresponding toanodes such as anode 214 shown in FIG. 14. (Anode 214 shown in FIG. 14differs from a conventional anode as described below.) Electrons arefirst generated by the hot filaments that form a cathode array 226. Apositive voltage is applied between the cathode array 226 and anodes214. When the control grid voltage is on, the electrons are acceleratedby the positive potential towards the phosphor layer which is deposited,in a conventional vacuum fluorescent display, on top of the anodes. Theremainder of the display conventionally comprises a glass faceplate 212,glass backplate 228, and a glass frit seal 222, containing a vacuum forthe control wire grid 224 and filament cathodes 226.

A resonant microcavity structure may be used to improve the performanceof this type of display. One possible illustrative embodiment isdepicted in FIG. 14. The resonant microcavity structure 217 comprisingan active layer 218 sandwiched between a pair of dielectric mirrors 216,220 disposed between the control wire grid 224 and the anode 214replaces the powder phosphor that is conventionally deposited on anodessuch as anode 214.

For small scale monochromatic displays, one resonant microcavity 217would be used and the pixels would be determined by the control grid 224and cathode 226 arrangement. If full color is required, a resonantmicrocavity 217 would be required for each primary color. An efficientlayout would comprise alternating stripes of microcavities 217 withseparate stripes for each color. In large screen applications, eachpixel would incorporate one resonant microcavity designed for a specificcolor. The array would then comprise a triad of red, green and bluepixels.

As discussed above for the two CRT embodiments, the parameters such asdirectionality, brightness, color and the microcavity structure applyfor the vacuum fluorescent display. The design considerations andmethodology required for optimizing the display are also the same. Forexample, since this display is a direct view type, with light emissiondirected towards a viewer in the direction indicated by arrow A, thedivergence of the emitted light would be tailored for the viewerdistance and required viewing angle. Incorporation of lenses anddiffusers must be considered. The design of vacuum fluorescent displaysfor specific applications is well known in the art.

In another embodiment using excitation by electrons, the resonantmicrocavity can be incorporated into field emission displays for bothprojection and direct view applications. This display operates on theprinciple of electrons tunneling from a microscopic tip or a microscopicregion of a low work function material. The electrons are thenaccelerated via a positive potential and penetrate an adjacent phosphorlayer. Typically there is a evacuated region between the tips and thephosphor, but in some applications the phosphor may be grown directly ontop of the emitting surface.

These displays may operate in both a high voltage and low voltage mode.In the high voltage application, typically above 500 volts, an emitterarray would be assembled behind each microcavity. The display couldconsist of one microcavity that is the size of an entire display togenerate monochromatic light or the display could consist of pixel sizemicrocavities suitable for producing color images. In these structures,the voltage must be sufficiently high that the electrons can pas throughthe bottom mirror of the RMD into the active layer to stimulate thephosphor.

FIG. 15 provides one illustrative embodiment of a monochromatic fieldemission display 230 incorporating a resonant microcavity 239 comprisingan active layer 236 between mirrors 234 and 238. When a positive highvoltage is applied between the anode 240 and cathode 246, the electronsare generated by the field emissive material 244, which is sealed withinan evacuated region 242 by seals 248. The electrons are then acceleratedthrough the evacuated region 242, penetrate the resonant microcavity 239and excite the active layer 236. The aluminum layer 240 is approximately50 nanometer thick and conducts the electrons to ground.

In the low voltage application, the field emissive material must belocated inside the RMD due to the limited penetration depth of the lowenergy electrons. Suitable emissive materials must have a low workfunction so that a low voltage applied to the material will inducesufficient number of electrons to be emitted. In this application, a lowvoltage is applied across the resonant microcavity and induces electronsto tunnel from the electron emissive material into the phosphor andexcite the activators. Under the influence of the applied field, theelectrons travel through the phosphor and then into another materialwhich conducts the electrons to ground.

In one illustrative embodiment of a low voltage field emission display250 illustrated in FIG. 16, the resonant microcavity 253 (which isdeposited on substrate 252) would comprise an oriented diamond filmlayer 256 that is deposited on one side of the phosphor layer 258.Another conductive film layer 260 similar to diamond film layer 256would be deposited on the opposite side of phosphor layer 258 to conductthe emitted electrons to ground. A low voltage potential would beapplied between conductive layers 260 and 256. Reflectors 254, 262 aredisposed outside the sandwich-like structure formed by conductive layers256, 260 and phosphor layer 258. This embodiment is depicted in FIG. 16.

In the case of the simple coplanar microcavity, the key designspecification in all the display applications is locating the activelayer at or near the antinode of the electric field inside the cavity Inthe low voltage field emission display, this specification is criticalgiven the thickness of the active layer The basic structure of aphosphor layer sandwiched between two emissive layers can be repeated,provided that the phosphor material is located at or near an antinode.An illustration of a standing wave field in one illustrative embodimentof a resonant microcavity display 300 is shown in FIG. 17. Microcavitydisplay 300 comprises a substrate 302, a pair of mirrors 304, and anactive layer 306. The electric field amplitude 310 in active layer 306is shown schematically. A node 311 and an antinode 312 are shown.

The design issues one must consider are fundamentally the same as hasbeen discussed in the other display applications. However, the index ofthe electron emissive material must now be a major factor in the designof the cavity. An additional concern is the choice of material for thespecific applied voltage range.

In addition, the RMD can be embodied in an electroluminescent display.In this display application, a RMD is sandwiched between two conductors.A voltage signal is applied to the conductors and thereby induces whatis termed thin film electroluminescence (TFEL). An array of pixel-sizeelements is constructed to form a luminescent screen creating a TFELflat panel display.

This embodiment would comprise an array of pixels, where each pixelwould be an electrically activated microcavity. FIG. 18 shows one pixelin the array 160. The pixel comprises a visibly transparent substrate162, a layer of Indium doped Tin Oxide (ITO) 164 (a transparent metal)acts as ground, and a resonant microcavity 166. The resonant microcavity166 comprises a front reflector 168, a phosphor-based active region 170and a back reflector 172. Disposed next to the back reflector 172 is analuminum layer 174, which is deposited on each microcavity in suchmanner that each cavity is electrically isolated.

This display would be excited by applying a voltage to the aluminumlayer 174 of the pixel microcavity 166. The addressing of pixels iscommon in the art of flat panel display design.

This display would be optimized by measuring the amount of useful lightemitted versus the electric field intensity. Particular attention mustbe paid to the phosphor selected since (in this embodiment) theelectroluminescence efficiency is important.

Also, the RMD could be embodied as an array of pixels in a flat paneldisplay which uses ultra-violet light to excite the phosphor. As seen inFIG. 19, each pixel 180 would comprise a plasma discharge lamp 182 thatgenerates ultra-violet light which passes through a back reflector 184and excites the active region 186 (i.e., the phosphor). the emittedlight then passes out of the display through the front reflector 188 andthe substrate 190.

The RMD concept can also be used to fabricate a transparent direct viewflat panel display. This display is visibly transparent except at thespecific resonant wavelengths of the microcavities that are used in thedisplay. Both monochrome and full color displays are possible. Forexample, to create a full color display, one could chose the threewavelengths that correspond to the three fully saturated colorsspecified by the international CIE color standard for red, green andblue.

The transparent property is created by fabricating resonantmicrocavities that use reflectors that only function as high efficiencymirrors within a narrow wavelength bandwidth, typically one nanometer orless. Outside this region, the reflectors transmit nearly 100% and thusthe RMD appears transparent to the eye. Such narrow band reflectors canbe best built using a multiple cavity structure employing dielectricmirrors.

In one illustrative embodiment shown in FIG. 20, a flat panel displaywould consist of an array of pixel size RMDs 500 excited by an electricfield. Two transparent electrodes 504, 514 must be connected to eitherside of each microcavity 506 and could be best fabricated using IndiumTin Oxide (ITO). Microcavity 506 itself would comprise an active layer510 between mirrors 508, 512.

In addition to creating a transparent display, the same reflectorstructure can be used to create a high contrast display. In thisembodiment, the rear surface is made opaque by another opaque layer (notshown) or by replacing ITO layer 514 with an opaque conductor. Externalambient light would be transmitted through the display and then absorbedby the rear layer. The reflection from the front surface would beminimized because of the high transmission properties of the reflectorsoutside the resonance wavelengths. Such a display can be made to havevery high contrast ratios on the order of a 100 or greater. These directview displays can use any of the three excitation sources.

The use of organic materials permits the construction of a RMD out offlexible materials such as plastics.

The resonant microcavity display can also be excited using laser light.Laser light results from stimulated emission processes and isdistinguished from spontaneous emitted light by the high degree ofspatial and/or temporal phase coherence. The laser light would be chosento have a wavelength that is absorbed by the phosphor. The cavitystructure must be designed to pass the laser wavelength. In oneembodiment shown in FIG. 21, a laser 412 would be scanned horizontallyand vertically across a luminescent screen 401 in a manner similar tothe electron beam in a cathode ray tube. The steering of beam 410 istypically accomplished by rotating mirrors and acoustic opticmodulators. The ability to write sequential information with lasers iswell known in the art. Luminescent screen 401 itself comprises substrate402 and microcavity 403, including mirrors 404, 408, and active layer406.

The RMD could also be used in a reverse configuration to absorb lightand generate an electric signal. The physics that yields the enhancedemission of light demonstrated in the above display also producesenhanced absorption. The light energy has to be converted into electricenergy.

Another application of the resonant microcavity using its property ofenhanced absorption is in field of photography. In this application, thefilm would comprise resonant microcavities in which the active layerincludes a photosensitive material. As a result, this film would absorbonly at certain wavelengths corresponding to the three primaries. Sincethe amount of absorption can be precisely controlled, the film would becapable of extremely accurate color reproduction. Information could alsobe recorded by deriving an electrical signal from the photosensitivematerial within the microcavity. The general design would be similar todigital cameras employing charge coupled detectors.

The unique ability of an RMD to influence the emission characteristicsmay also be used in memory storage devices. As explained earlier, theconfinement of an optical material in a resonant microcavity affects thedecay rate. Depending on whether the cavity is in resonance with thetransition energy of the optical material or not, the lifetime is eitherdecreased or increased. It is therefore possible to significantlyenhance the lifetime of the material and to use this effect to storeinformation.

Another possible way to store information with a resonant microcavitywould be based on hole burning. This process and its application for thestorage of information is well known. By putting the material in aresonant microcavity one could not only use the enhanced absorption butalso the earlier described effect of increased lifetime to make the holeburning process more efficient.

RMDs could also be used in the design of light valves. This wouldrequire two RMD's. One RMD without a phosphor would be grown on top of aRMD with a phosphor. The first RMD would modulate the intensity of thelight emanating from the second RMD. The modulator would work by tuningthe first RMD to its resonant frequency or tune it away from itsresonant frequency. The process of tuning the first RMD (using theelectro-optic or the piezo-electric effect) would be achieved byapplying a voltage to the first RMD. This modulator could also be usedas a switch by turning the light completely on and completely off. Amodulated RMD 421 grown on substrate 422 is shown in FIG. 22. In thisfigure, RMD 421, comprising mirrors 424, 428 with active layer 426between, is modulated by applying a voltage V to mirror layers 424 and428. This modulation can be accomplished using either electro-optic orpiezo-electric effects.

The ability to tune the cavity resonance using, for example,electro-optic or piezo-electric effects, would allow the RMD to beutilized in a variety of communication modes. Resonant microcavitiescould be designed to emit light and receive light over a range offrequencies and solid angles. These frequencies and solid angles couldbe modified by applying electric signals. Thus RMDs could be used tosend and receive information. Friend or foe identifiers used in militaryequipment would be one possible use.

Using RMD's in a Plasma Display Panel could also be used to build afluorescence lamp. Compared to common fluorescence lamps the RMD lamphas the advantage of strongly enhanced fluorescence which results in agreater efficiency. A single RMD lamp would emit light of a certainwavelength. This is useful for applications such as stage-lamps. Commonstage lamps emit over the UV, the visible and the infrared region anduse filters to select a certain wavelength (color). This filter-processmakes the lamp very inefficient since most of the light is not allowedto exit the lamp. In contrast, the RMD lamp creates only light of acertain wavelength and does therefore not require a filter. Theefficiency is therefore much higher. The combination of a R, G and Bdevice would result in a white light source.

In general, any light source can in principle be substituted by theresonant microcavity display. For example, incandescent lights aretypically filtered to produce colored lights for car tail lights andtraffic signal lights. Resonant microcavities can replace these currentlight sources with highly efficient single color and directional lightsources. Excitation could use any of the means already discussed.

In non-emissive displays, the light source and image producing surfaceare separate. The image is typically formed using a light valve whichmodulates the light produced by the light source. A common light valvedisplay uses a combination of one or more liquid crystals and polarizersand forms what is called a liquid crystal display (LCD). Light valvesare used in both reflection and transmission and find use in bothprojection and direct view applications. The pixel size is determinedsolely by the light modulator.

In each application, a sufficiently bright light source is required.Often the display also requires full color capability. Currently forflat panel application, a fluorescence lamp is used as a backlight andcreates the white light that is then modulated by a LCD panel. To createa full color flat panel display, color filters are inserted at eachpixel to filter the white light and generate the three primary colors.

The RMD can be incorporated in such flat panel display applications andform the light source. For a monochromatic display, the modulator wouldbe attached to one large area resonant microcavity. The microcavity canbe excited by any one of the three excitation means. Full color would bebest generated by an array of microcavities consisting of alternatingstripes in which each striped region is constructed to form onecontinuous resonant microcavity designed to generate one color.

For projection devices, an arc lamp is used to generate a white lightsource and the color is typically generated by using dichloric filtersto separate the three primary color components of the white light.Instead, the three colors can be produced by three independent resonantmicrocavities or by producing an array of microcavities.

In addition, a LCD modulator requires the input light to be initiallypolarized and uses a polarizer located in the input. It is possible toeliminate this polarizer by designing the resonant microcavity togenerate polarized light. This can be accomplished in a number of ways.For example, the region between the mirrors can be fabricated usingbirefringent material in such a manner the cavity will resonate atdifferent frequencies depending on the polarization of the light. Thecavity can be designed so that only one polarized light component willresonate at the desired frequency.

The principal advantage of using resonant microcavities to generate thelight used for light modulators is the increased light outputefficiency. The RMD light source will produce high brightness levels andis highly directional. The latter is particularly useful for LCDapplications since the input light must be contained within a certainrange of solid angle. In addition, the elimination of color filters anddichroic beam splitters will increase the overall throughput. The otherengineering advantage is the compact nature of the RMD which isparticularly useful for flat panel applications.

In one illustrative embodiment shown in FIG. 23, a monochromatic flatpanel display 270 is depicted. In this example, the resonant microcavity275 is excited by UV light generated by a plasma discharge 282 excitedby a source 284 of AC power. Any damaging UV that leaks out pastmicrocavity 275 is absorbed by substrate 274; another UV blockingsubstrate 286 may also be used on the other side of plasma discharge282. The light valve uses an LCD 272 to modulate the light. LCD 272 canbe addressed in a number of modes and this specification is not affectedby using the resonant microcavity. The key design considerations for themicrocavity would involve the divergence of the light, the lightpolarization, brightness and resonance wavelength.

The above embodiments are given as illustrative examples and are notintended to impose any limitations on the invention.

What is claimed is:
 1. A luminescent display comprising: a plurality ofresonant microcavities each with an active region, wherein each activeregion has a phosphor disposed therein for emitting light; some of saidplurality of resonant microcavities have a phosphor disposed therein foremitting red light; some of said plurality of resonant microcavitieshave a phosphor disposed therein for emitting green light; and some ofsaid plurality of resonant microcavities having a phosphor disposedtherein for emitting blue light.
 2. The luminescent display of claim 1wherein each microcavity comprises: (a) a substrate; and (b) a structuredisposed upon said substrate comprising said active region and aplurality of reflective regions.
 3. The luminescent display of claim 2wherein the plurality of reflective regions comprise: (a) a frontreflective region disposed upon said substrate, and (b) a backreflective region; and the active region is disposed between the frontand the back reflective regions.
 4. The luminescent display of claim 3in which the front reflective region, the active region, and the backreflective region comprise thin films.
 5. The luminescent display ofclaim 2 wherein said reflective regions comprise dielectric reflectors.6. The display of claim 1 comprising: an first electron gun for excitingthe microcavities having a phosphor disposed therein for emitting a redlight; an second electron gun for exciting the microcavities having aphosphor disposed therein for emitting a green light; and an thirdelectron gun for exciting the microcavities having a phosphor disposedtherein for emitting a blue light.
 7. The display of claim 1 wherein:said red light emitting resonant microcavities, said green lightemitting resonant microcavities and said blue light emitting resonantmicrocavities alternate.
 8. The display of claim 1 wherein: saidplurality of microcavities alternate in a pattern with a red lightemitting resonant microcavity, followed by a green light emittingresonant microcavity, followed by a blue light emitting resonantmicrocavity; and said pattern repeats throughout the display.
 9. Thedisplay of claim 1 wherein: said microcavities are pixel sizemicrocavities.
 10. A luminescent display comprising: a first pluralityof resonant microcavities each with an active region, wherein eachactive region has a phosphor disposed therein for emitting red light; asecond plurality of resonant microcavities each with an active region,wherein each active region has a phosphor disposed therein for emittinggreen light; and a third plurality of resonant microcavities each withan active region, wherein each active region has a phosphor disposedtherein for emitting blue light.
 11. The display of claim 10 comprising:an first electron gun for exciting the microcavities having a phosphordisposed therein for emitting a red light; an second electron gun forexciting the microcavities having a phosphor disposed therein foremitting a green light; and an third electron gun for exciting themicrocavities having a phosphor disposed therein for emitting a bluelight.
 12. The display of claim 10 wherein: said red light emittingresonant microcavities, said green light emitting resonant microcavitiesand said blue light emitting resonant microcavities alternate.
 13. Thedisplay of claim 10 wherein: said pluralities of microcavitiesalternative in a pattern with a red light emitting resonant microcavity,followed by a green light emitting resonant microcavity, followed by ablue light emitting resonant microcavity; and said pattern repeatsthroughout the display.
 14. The display of claim 10 wherein: saidmicrocavities are pixel size microcavities.